Since a lithium ion secondary battery in which a lithium-containing transition metal oxide such as lithium cobaltate is used as a positive electrode, and carbon material which is capable of doping and dedoping lithium is used as a negative electrode, has a high energy density, it is prevalent as a power source for portable electronic appliances represented by a cellular phone. As a power source mounted on a hybrid vehicle, a lithium ion secondary battery having a high energy density is also attracting attention. Along with the popularization of portable electronic appliances and hybrid vehicles, demand for lithium ion secondary batteries is ever-increasing.
A lithium ion secondary battery generally includes a laminated body of a positive electrode, a separator containing an electrolyte, and a negative electrode. A principal function of the separator is to prevent a short circuit between the positive electrode and the negative electrode. The separator is required to have properties such as lithium ion permeability, mechanical strength and durability.
At present, as a film suitable for a separator for a lithium ion secondary battery, a large number of polyolefin microporous membranes have been proposed. Since a polyolefin microporous membrane satisfies the above-mentioned properties and has a shutdown function, it is widely used for a separator for a lithium ion secondary battery. In the field of non-aqueous secondary batteries, the shutdown function means a function in which, when the temperature of the battery abnormally increases, polyolefin melts and holes of a porous membrane are blocked, thereby blocking an electric current, and the shutdown function works for preventing thermal runaway of the battery.
However, even when the shutdown function works and the electric current is temporarily blocked, when the temperature inside the battery increases above the melting point of polyolefin constituting a microporous membrane, the whole polyolefin microporous membrane melts (i.e., meltdown). As a result, a short circuit occurs inside the battery, by which a large amount of heat is generated, and the battery may emit smoke, catch fire or explode. For this reason, the separator is required to have, in addition to the shutdown function, a heat resistance such that the separator does not melt down even at a temperature higher than the temperature at which a shutdown function is exerted.
Therefore, in Patent Document 1, a separator for a non-aqueous secondary battery is proposed in which the surface of a polyethylene microporous membrane is covered with a heat resistant porous layer including a heat resistant polymer such as a fully aromatic polyamide. In Patent Document 2, a separator for a non-aqueous secondary battery is proposed in which inorganic particulates such as alumina are contained in a heat resistant porous layer, to improve heat resistance as well as the shutdown function. In Patent Document 3, a separator for a non-aqueous secondary battery is proposed in which metal hydroxide particulates such as aluminum hydroxide are contained in a heat resistant porous layer, to improve flame resistance as well as heat resistance. In these separators for a non-aqueous secondary battery, both of the shutdown function and the heat resistance can be attained, and excellent effects can be expected in safety of the batteries.
However, in the separator for a non-aqueous secondary battery which has the structure in which a polyolefin microporous membrane is covered with a heat resistant porous layer, the shutdown function which the polyolefin microporous membrane exhibits tends to be restrained. On the other hand, when the composition of the polyolefin microporous membrane is made such that flowability of the polyolefin is high in order to improve the shutdown function of the polyolefin microporous membrane, a problem arises in that the mechanical strength of the polyolefin microporous membrane decreases and, as a result, the mechanical strength of the separator for a non-aqueous secondary battery decreases.
Recently, from the viewpoint of increase in the capacity of a lithium ion secondary battery, a variety of high-capacity type positive electrode materials and negative electrode materials have been developed. However, in the high-capacity type positive and negative electrode materials, volume change during charging and discharging is relatively large and, as described below, the battery properties may decrease depending on the volume change of the electrode.
Since the separator is disposed between the positive electrode and the negative electrode in the battery, a compressive force or a restoring force acts in the thickness direction of the separator due to expansion and shrinkage of the electrode accompanying charging and discharging of the battery. In the case of low-capacity type positive and negative electrode materials such as conventional lithium cobaltate or hard carbon, since the volume change of the electrode is small, the deformation of the separator in the thickness direction is also small, and the battery properties are not particularly affected. However, in the case of using an electrode material which has a large ratio of volume change during charging and discharging, the acting force from the electrode to the separator becomes large. Further, when the separator cannot follow the volume change of the electrode and the porous structure of the separator cannot recover from a compressed state, a phenomenon in which a sufficient amount of electrolyte cannot be retained in the holes of the separator, that is, a liquid depletion phenomenon may occur. This liquid depletion phenomenon may consequently deteriorate the repeated charging-discharging property (cycling property) of the battery.
In order to solve the above-mentioned liquid depletion problem, controlling of physical properties such as elasticity of the polyolefin microporous membrane may be thought of. However, when a certain physical property of the polyolefin microporous membrane is controlled, other physical properties are necessarily also affected. As described above, since good shutdown properties and mechanical strength are also demanded for the polyolefin microporous membrane, a technique in which these various properties can be improved with good balance is also demanded.